It's not the best day for solar and wind in California, it would seem: it's cloudy even in the desert, and there isn't much wind across the state. But the weather's always the same underground: geothermal power plants plug along delivering around 20 gigawatt-hours of power to the California grid every single day. Why aren't we taking more advantage of this 24-7 power source?

It's a fair question. Even on days with lots of wind and sunshine, California's geothermal plants often provide more power to the grid than any other renewable source, or at least come in in second place behind wind turbines. The constant flow of this renewable would seem to offer a solution to that whole "intermittent renewables" dilemma that's prompting hundreds of millions of dollars worth of research into high-tech battery banks and costly solar thermal storage solutions. And yet while wind and solar installations have grown several times over in the last few years, geothermal output in California hasn't grown at all.

Part of the reason is that geothermal power plants literally do offer a solution: a solution of potentially hazardous materials that emerges from the reservoirs of geothermally heated water deep underground. The fluid that emerges from these reservoirs can include heavy metals, arsenic, and boron, methane, hydrogen sulfide and carbon dioxide, and even radioactive elements. The evaporates from this briny fluid, called "filter cake" in the geothermal trade, are regulated as hazardous waste. Imperial County geothermal plants produce between between 40,000 and 60,000 tons of this waste each year.

One person's hazardous waste is another person's ore deposit. Geothermal brine often includes potentially valuable minerals such as lithium that could conceivably be extracted and used in related industry. And comparing geothermal's waste stream to coal-fired plants makes geothermal look pretty good. Still, the hazardous waste issue does make siting geothermal facilities a potentially fraught matter in a political sense.

There's another aspect to the siting problem: geothermal doesn't work everywhere. Right now, to build a geothermal plant, you need three things:

Geothermal heat close enough to the surface that you can reach it;

An area of fractured rock in that heated zone to provide reservoirs for geothermal water and steam;

Water to fill those reservoirs.

Finding all three of those prerequisites in the same place isn't always easy, and at present geothermal power is only viable where they naturally coincide -- namely, in areas with hot springs and geysers. That's why seismically active Nevada and California are national leaders in geothermal power production. Even within California, geothermal is more or less limited to a few areas: the north end of Wine Country, the Imperial Valley, the general vicinity of Mammoth Hot Springs.

Seattle-based startup AltaRock is working to develop ways of creating geothermal reservoirs where they don't already exist, by using a technology similar to fracking -- and then injecting recycled plastic to create barriers within the reservoirs they've created. That creates multiple reservoirs from one new one, theoretically allowing more power plants to harness the newly opened deep-earth cracks.

If AltaRock's technology proves successful, it may prove a huge boon to the geothermal industry -- and will likely open up whole new arenas for public opposition due to siting and seismic concerns.

We'll be watching such experiments with interest. Even if new technology opens up new geothermal territories, however, the field will continue to suffer one distinct disadvantage over wind and solar: modularity. It's easy to add relatively small amounts of solar to the grid. Solar power generating capacity can be connected to the grid a few hundred watts at a time. Individual wind turbines have capacities of 2.5 megawatts or less. While geothermal plants don't have to be as large as coal-fired plants to be practical -- plants of less than 50 megawatts aren't unheard of -- it's impractical to add a megawatt or two of geothermal at a time. The plants are more complex than wind or solar photovoltaic facilities, what with deep drilling and high-pressure steam to be piped into turbines and reinjected into the earth.

It's no wonder that wind and solar have seen significantly greater attention in California: they're the low-hanging fruit. For the long term, though, it's clear that geothermal's basic reliability means it has a crucial part to play in balancing California's grid as we wean ourselves from fossil fuels.

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